Laser units including excimer laser, TEA-CO.sub.2 laser, TEMA-CO.sub.2 laser or copper vapor laser as well as accelerators such as a linear induction accelerator generally incorporate a high repetition rate high voltage pulse generators, in which the energy stored in a capacitor is discharged with a discharge tube such as a thyratron or a semiconductor switching device such as a thyristor.
For improving the output power, repetition frequency, efficiency and reliability of such high voltage pulse generators, it is important to reduce losses at above switching devices. For this purpose, magnetic components such as a step-up transformer, a saturable transformer and a saturable reactor are used.
The above linear induction accelerator incorporates an acceleration cavity utilizing a magnetic core for generation or acceleration of charged particle beam such as electron beam.
The ion sources in a neutral beam injector comprise magnetic components to suppress surge voltage.
For the magnetic components used for these applications, it is important to reduce magnetic core volume and core loss. It is well known that the magnetic core volume and the core loss are in inverse proportion to the square of the effective magnetic flux density swing .DELTA.B, when the temperature rise at the magnetic core caused by the core loss is ignored. If the reset magnetizing force is large enough, the magnetic flux density swing.DELTA.B is about twice of the effective magnetic flux density Bms. Thus, it is preferable to use a magnetic core made of Fe-based soft magnetic alloy with high saturation magnetic flux density.
In the aforementioned applications of magnetic components, the magnetization speed .DELTA.B/.tau. reaches to 0.1 to 100 T/.mu.s. In case that the temperature rise at the magnetic core due to eddy current loss cannot be ignored when a material with low electric resistivity such as Fe-based soft magnetic alloy is used, insulation oil or insulation gas is usually used to suppress the temperature rise of the magnetic core to a practically allowable temperature. If, however, the eddy current loss of the magnetic core is too large, the temperature rise of the magnetic core cannot be sufficiently suppressed and the efficiency of the unit is largely spoiled.
There are two methods to obtain a magnetic core with low eddy current loss using soft magnetic alloy: One is to use ribbons of soft magnetic alloy to form a toroidal core or a stacked magnetic core; the other is to use powder of soft magnetic alloy to be formed into a magnetic core under pressure. However, the latter magnetic core generally has a relative permeability of as low as several hundreds or less. Accordingly, the former magnetic cores with soft magnetic alloy ribbons are mainly used for the applications of this invention.
It is known that, for lower eddy current loss, the magnetic core needs to be comprised with thin soft magnetic alloy ribbons with high resistivity and to have insulation coating formed on their surface.
Various toroidal magnetic cores have been used for this purpose: toroidal cores comprising heat-treated Fe-based amorphous soft magnetic alloy ribbons and insulation films such as polyethylene Terephthalate films wound together, the cores formed with the above soft magnetic alloy ribbons and polyimide films wound together and then heat-treated, the cores comprising the heat-treated soft magnetic alloy ribbons on which Polyimide insulation films are formed before winding, or the cores made of the soft magnetic alloy ribbons having ceramic insulation coating comprising Al.sub.2 O.sub.3, SiO.sub.2 or MgO on the surface.
However, the saturation magnetostriction constant of the Fe-based amorphous soft magnetic alloy ribbons are as large as about 20.times.10.sup.-6 or more. Therefore, unless the ribbons are provided with MgO or colloidal silica insulation coating of about 0.3 .mu.m thick applied or SiO.sub.2 insulation coating by deposition method of about 0.2 .mu.m thick formed on the surface, the effective magnetic flux density Bms or the effective residual magnetic flux density Brms in the direct current magnetic characteristics of the ribbon is deteriorated under the effect of the stress applied onto the ribbon during winding with insulation films or insulation coating formed on the ribbon surface.
On the other hand, the ribbons with about 0.3 .mu.m thick insulation coating of MgO or colloidal silica and the ribbons with about 0.2 .mu.m thick SiO.sub.2 insulation coating by the deposition method are known to have insufficient insulation characteristic under the operation condition where the magnetization speed .DELTA.B/.tau. is about 0.1 to 100 T/.mu.s.
If the above MgO or colloidal silica insulation coating is made thicker to improve the insulation characteristic, the bonding strength between the ribbons and the coating materials is reduced, which impedes stable performance in practical use of the core. Besides, for the SiO.sub.2 insulation coating by the deposition method, thicker film for an improved insulation characteristic is not preferable from the viewpoint of production efficiency.
Japanese Patent Application Laid-Open No. 302504/1988 or No. 20444/1991 discloses a nano-crystalline soft magnetic alloy ribbon in contrast with above materials. According to these inventions, a ceramic insulation coating is formed onto an amorphous alloy ribbon, then the ribbon is heat treated at a temperature over its crystallization temperature, so that nano-crystalline particles having diameter of 50 nm or less represent at least 50% of the structure. The value of the saturation magnetostriction constant for such ribbon is smaller than that for an Fe-based amorphous soft magnetic alloy ribbon by one digit or more.
Thus, according to Japanese Patent Application Laid-open No. 297903/1990, toroidal magnetic cores with nano-crystalline soft magnetic alloy can be made by heating the cores and coated films comprising mixture of silanol oligomer and micro ceramic particles to form a ceramic insulation coating with cross-linked silanol oligomer serving for layer insulation. The above mentioned magnetic cores with ceramic insulation coating have almost the same direct current magnetic characteristics as the ribbon itself. Its core loss when operated at a magnetization speed .DELTA.B/.tau. of several tens of T/.mu.s or more is known to be considerably smaller than that for the toroidal core with an insulation film formed on the Fe-based amorphous soft magnetic alloy ribbon.
However, the toroidal core as described above still have some drawbacks. Suppose a toroidal core formed by winding nano-crystalline soft magnetic alloy ribbons with the above SiO.sub.2 insulation coating thereon, which is heat treated at a temperature above the crystallization temperature of the ribbon under the direct current magnetic field of 800 A/m along its magnetic path direction. When such cores are subjected to the durability test where the cores are operated at a repetition rate of 500 Hz and for magnetic flux density swing .DELTA.B of 2.5 T and magnetization speed .DELTA.B/.tau. of 50 T/.mu.s (corresponding to 25 V for the inter-layer voltage), the loss at the magnetic core rapidly increases under application of pulse voltage for only about 10.sup.5 shots, because the core has only insufficient layer dielectric strength.
Magnetic cores used in laser units, accelerators or surge block cores usually operate at a magnetization speed .tau.B/.tau. of about 0.1 to 100 T/.mu.s. Assuming a soft magnetic alloy ribbon having a width W of 25 mm and a thickness t of 20 .mu.m is used to form a toroidal core, which is operated for magnetic flux density swing .DELTA.B of 2.5 T and at a magnetization speed .DELTA.B/.tau. of 50 T/.mu.s, resulting in uniform induction of voltage for the layers of the ribbon constituting the toroidal core. In this case, the pulse height value Vp of the inter-layer voltage induced between layers of the toroidal core is 25 V/layer according to the formula (1). EQU Formula: Vp.gtoreq.(W.multidot.t.multidot..DELTA.B)/.tau. (1)
The nano-crystalline soft magnetic alloy ribbons described here are manufactured by the rapid quenching method generally referred to as single roller method. And the toroidal core is formed by winding the ribbons with the insulation coating thereon and then heat-treated at a temperature above its crystallization temperature.
The ribbon manufactured by the single roller method as above generally has ten-point average roughness Rz of about 3 .mu.m according to JIS B0601 on its surface. Because of the effect of the surface roughness, the dielectric strength of the insulation coating becomes lower. Considering such deterioration of the dielectric strength, the insulation coating should be formed so as to resist against the value determined from the above formula (1). Besides, unlike usual dielectric conditions, it must be taken into account that the electric field intensity at the edges of the magnetic ribbon ends becomes higher than at its center when the actual toroidal core is operated under large amplitude of the magnetic flux density.
In general, to realize a highly reliable pulse generator, laser unit or accelerator system, the magnetic components are required to have stable magnetic characteristics even after the severest operations where, at a magnetization speed .DELTA.B/.tau. of 50 T/.mu.s, at least 10.sup.5 shots or more, or more preferably 10.sup.9 shots of pulse voltage is applied.
To attain magnetic cores whose change of the magnetic properties with time is limited to a ignorable level for practical use even after 10.sup.6 shots of pulse application or more at a magnetization speed .DELTA.B/.tau. of 50 T/.mu.s, it is required to form an SiO.sub.2 insulation coating having an average thickness of about 3 .mu.m or more on the ribbon surface when the above nanocrystalline soft magnetic alloy ribbons having width W of 25 mm, thickness t of 20 .mu.m and ten-point average roughness Rz of about 3 .mu.m are used for the magnetic cores.
In case of the nano-crystalline soft magnetic alloy ribbons, the value of the saturation magnetostriction is small (order of 10.sup.-6) and their magnetic characteristics are less deteriorated under the effect of stress. However, when a ceramic insulation coating with an average thickness of about 3 .mu.m, which amounts to 20% of the thickness of the ribbon, is formed on the surface, the effective magnetic flux density Bms or the effective residual magnetic flux density Brsm in the direct current magnetic characteristics may be lowered by the effect of the stress inevitably applied to time ribbon during insulation coating formation. Further, the relative permeability may be reduced and the core loss may increase during pulse voltage operation.
The nano-crystalline soft magnetic alloy ribbon is known to have a smaller volume in crystallized state than in amorphous state. If the insulation coating formed on the ribbon surface in amorphous state has an average thickness of about 3 .mu.m, such reduction of volume causes cracks or other defects in the insulation coating and leads to reduced bonding strength with the ribbon, which may result in peeling off from the ribbon surface.
If a toroidal core with defects in the ceramic insulation coating or with a reduced bonding strength between the insulation coating and the ribbon is operated at a magnetization speed .DELTA.B/.tau. of about 0.1 T to 100 T/.mu.s, the magnetostrictive vibration at the magnetic core generated in the operation promotes crack growth or peeling for the inter-layer insulation coating, which gradually reduces the inter-layer dielectric strength. This may cause a rapid change of magnetic characteristics of the core under the effect of pulse voltage of only about 10.sup.5 shots.